Equipment for use in molten metal plating bath

ABSTRACT

An equipment for use in a molten metal plating bath, comprising a bearing disposed in the molten metal plating bath for supporting a roller with an end, the bearing containing 70-95 wt. % of Si 3  N 4 , 2-10 wt. % of Al 2  O 3 , 1-10 wt. % of AIN and 1-10 wt. % of Y 2  O 3  and having a concave with a rectangular shape in section, the end of the roller being directly disposed in the concave to be supported by the bearing without using rollers.

TECHNICAL FIELD

The present invention relates to various types of equipments for use inbaths employed in plating of non-ferrous metals other than copper, suchas, for example, molten zinc plating, molten aluminum plating, orplating of molten zinc-aluminum alloys; in particular, the presentinvention relates to various types of equipments used in baths which aredisposed within molten metal plating baths having a composition suchthat silicon nitride (Si₃ N₄) is the main component thereof (forexample, bath rollers [support rollers, synch rollers, and the like],roller bearings, roller arms, melting boxes, snouts, pumps within baths,thrust locks, and the like).

BACKGROUND ART

Conventionally, materials resistant to melting and metal corrosion suchas low carbon steel, high chromium steel, and SUS316L, and the like,were commonly employed for various types of equipments used in bathsemployed in molten zinc plating, molten aluminum plating, or the platingof molten zinc-aluminum alloy.

In a case of requiring greater resistance to corrosion than that offeredby these materials resistant to melting and metal corrosion, it iscommon to build up metal by welding or to form a film by flame sprayingon the surfaces of these metal implements and equipments using one or amixture of two or more of oxides, borides, nitrides, carbides,fluorides, and the like of various corrosion resistant metals such as W,Co, Mo, and Cr and the like. In this case, the service life during whichsuch implements or equipments could be continuously employed withoutcausing defects in the high grade products was within a range of 15-90days.

In order to improve the productivity of the manufacturing industry, itis necessary to operate facilities over a long period of time withoutaffecting the quality of the materials produced.

In order to respond to this need in molten metal plating operations, itis necessary that the service life, during which the implements andequipment used within the plating bath can be continuously employed, beas long as possible, in addition to management of the plating baths.

The present invention solves the problems in the conventional technologydescribed above; it is an object thereof to provide various types ofequipments for use in molten metal plating baths which are capable ofstable use over a long period of six months or more.

DISCLOSURE OF THE INVENTION

In order to attain the above object, the present inventors haveconducted diligent research, and as a result discovered that it iseffective to form various equipments for use in molten metal platingbaths using a composition having as a main component thereof siliconnitride, which is a material which does not react with the molten metaland which is easily worked; the present invention was completed based onthis discovery.

The invention based on this discovery comprises equipments used inbaths, such as rollers, bearings, roller arms, melting boxes, snouts,pumps, thrust locks, and the like, which are disposed within moltenmetal plating baths, and which is formed by a composition containing Si₃N₄ : 70-95%, Al₂ O₃ : 2-10%, AlN: 1-10%, Y₂ O₃ : 1-10%, and unavoidableimpurities.

BRIEF DESCRIPTION OF THE DIAGRAMS

FIG. 1 is a cross-sectional view showing a molten metal plating deviceemploying a roller in accordance with the present invention.

FIG. 2(a) is a side view showing an embodiment of a bearing for asupport roller in accordance with the present invention.

FIG. 2(b) shows a front end of an axle shown in FIG. 2(a).

FIG. 3 is a side view showing another embodiment of a bearing for asupport roller in accordance with the present invention and across-sectional view of this bearing.

FIG. 4 shows a bearing combined with the synch roller bearing part ofthe present invention.

    ______________________________________                                        Description of the References                                                 ______________________________________                                        1          plating steel plate                                                2             molten metal bath                                               3             snout                                                           4             synch roller                                                    5             synch roller holder                                             6             support roller                                                  7             support roller holder                                           8             melting box                                                     9             oven                                                            10           support roller cylinder                                          11           axle provided with a screw-shaped groove                         12           axle end surface groove                                          13           projecting axle end surface                                      14           bearing                                                          15           synch roller                                                     16           synch roller axle                                                17           synch roller arm                                                 18           synch roller bearing                                             19           synch roller thrust lock                                         ______________________________________                                    

BEST MODE FOR CARRYING OUT THE INVENTION

The structure and function of the present invention will now beexplained.

The various types of equipments for use in molten metal plating bathshaving an increased service life in accordance with the presentinvention are formed using a composition having as a main componentthereof silicon nitride, which is non-reactive with the molten metal,and which is easily worked, so that it may be industrially employed.

In order to compensate for the defects of silicon nitride, that it isdeficient in shock resistance and in strength, it is commonly combinedwith metals; however, there are great differences in the thermalexpansion coefficients of metal and ceramics, and so there has beenlittle success in the use thereof for various types of equipment used inmolten metal plating baths.

In the present invention, the formation is conducted using a solelyceramic composition having silicon nitride as the main ingredientthereof, and the design is such that sufficient shock resistance andstrength are exhibited.

That is to say, in case a roller support axle and a roller bearingformed with a ceramic composition having silicon nitride as the chiefcomponent thereof come into contact and rub against one another and asufficient service life can not be obtained, a spiral groove is providedon the surface of the roller support axle, and one or more grooves aresupplied on the end surface of the support axle, and by means of thesegrooves, molten metal enters into the gap between the surface of theroller support axle and the inner surface of the bearing, and these donot directly come into contact, so that the molten metal serves as alubricant.

Commonly, in rotating axle bearing which employs ceramics such as metaloxides, metal carbides, metal borides, metal fluorides and the like, theceramics are used for balls or needles, and structures are known inwhich metal is used on the inner race or the outer race, or the oppositecombination is employed.

There are chiefly two problems with the case described above. One ofthese is that although the parts formed from ceramics contribute to anincrease in service life, the metal portions have the conventional shortservice life, and do not contribute to the increase in service life ofthe implements and equipments which is the object of this invention.Another drawback is that dross having an extremely high hardness,comprising an alloy of two or more metals, is in suspension in themolten metal plating bath, and this enters in between the balls orneedles and the race, creating resistance and stopping the function ofthe bearing.

In order to avoid a stoppage in bearing function resulting from theentry of the dross, a sliding bearing has been proposed, and varioussuch bearings have been tested or partially produced. Ceramics areaffixed in some way to a portion of a bearing which is formed frommetal, that is to say, to the portion of the bearing which is contactwith the axle. A disadvantage in this case is in the same manner asabove, and the metal portions are eroded, so that the ceramic basematerial is eliminated, and no overall increase in service life can berealized.

The present invention solves all the deficiencies mentioned above;herein, the bearing comprises a sliding bearing formed in a unitarymanner from ceramics, and in the same way, in the case of the roller,the present invention makes it possible not merely to use ceramics atthose portions which are in contact with the bearing and whichexperience extreme abrasion and those portions which are in contact withthe steel plates and also experience extreme abrasion, but to use theunitary formation of the roller using ceramics. The equipment for use inmolten metal plating baths of the present invention does not employ acombination of metal and ceramics, so that there is no crackingresulting from the tensile or compressive forces generated bydifferences in thermal expansion coefficients, and use over a longperiod of time is possible.

The reason for limiting the composition ranges of the ceramic componentused to form the equipment for use in molten metal plating baths of thepresent invention will now be explained.

Si₃ N₄ : 70-95%

This compound is resistant to corrosion by nonferrous molten metalsother than copper, and is also resistant to acidic and alkalineconditions. Sintering formation is made possible by adding small amountsof Al₂ O₃, Y₂ O₃, and the like. At amounts of less than 70%, the thermalresistance and resistance to corrosion are insufficient, while when theamount is in excess of 95%, sintering formation becomes impossible.##EQU1##

By adding both of these together, the sintering formation of Si₃ N₄which is the chief component becomes possible. At amounts less than thelower limit, sintering becomes insufficient, while at amounts greaterthan the upper limit, practical application becomes impossible as aresult of the decline in thermal resistance and resistance to corrosion.

AlN: 1-10%

This compound has a high thermal conductivity and a high hardness, andserves to increase the strength. At amounts of less than 1%, theseeffects can not be realized, while at amounts of more than 10%, thebrittleness increases, and shock resistance declines.

When substances or elements other than these four are included asunavoidable impurities, the effects of the present invention will beundisturbed if the total amount is less than 1%.

An embodiment of the present invention will now be discussed; however,the present invention is not limited to this embodiment.

Embodiment 1

87% Si₃ N₄, 3% AlN, 5% Al₂ O₃, and 5% Y₂ O₃ by weight were mixed inpowdered form (having a grain size of 1.2 micrometers), 0.5 parts perweight of polyvinyl butyral were added as a binder to 100 parts perweight of the raw material powder, and this was agitated to produce amixture. This mixture was then press-formed at a pressure of 1,000kg/cm², and this was sintered for a period of 5 hours in a nitrogenatmosphere at a temperature of 1750° C., and the roller (having adiameter of 250 mm, a length of 1,900 mm, an axle diameter of 90 mm, anaxle length of 150 mm, and an axle screw depth of 2 mm, and a pitch of26 mm) and the bearing (having an inner diameter of 93 mm, an outerdiameter of 140 mm, and a length of 120 mm) shown in FIG. 3 wereobtained. The characteristics of the ceramic roller and bearing aftersintering are as shown below.

    ______________________________________                                        Flexural strength                                                                            100 kg/mm.sup.2                                                Surface hardness HV                                                                           1,580                                                         Resistance to corrosion                                                                           Erosion loss: 1 mg/month or less                          (test solution: test piece test                                               using a molten zinc bath)                                                     Resistance to abrasion                                                                        Amount of abrasion: 4 g/month or less                         (test conditions: an installed                                                support roller bearing)                                                       ______________________________________                                    

The roller and bearing obtained in embodiment 1 were incorporated intothe plating device of FIG. 1 (a molten zinc bath having a temperature of470° C.), and this was operated continuously for a period of one monthwith respect to cold rolled steel plates (type of steel: low carbonaluminum killed steel, physical properties: Yp 28 kgf/mm², Ts 36kgf/mm²) having an average width of 1,200 mm and an average thickness of1.25 mm at an average pass through speed of 115 m/min, and aftermaintenance, the roller of the present invention was used six timesunder the same conditions, and no defects were produced in the surfaceof the roller.

For the purposes of comparison, hard chromium plating having a thicknessof 30 micrometers was executed on a roller axle and bearing comprisingSUS316L having the same size as that of embodiment 1, and this wassubjected to plating operations under the same conditions. It wasnecessary to change the roller after a period of ten days from theinitiation of operations because of the rough quality of the rollersurface.

As described above, the equipment of the present invention exhibits anextremely long service life in the case of rollers, roller bearings, andthrust locks which are subjected to abrasion and rubbing; those portionswhich do not experience rubbing and abrasion, such as roller arms,melting boxes, and snouts have a semi-permanent service life. Whenconventional technologies are applied to such equipment, the surfacelife thereof is 2-3 years, with the exception of the oven.

Industrial Applicability

The present invention is structured as described above, so that theequipment for use in molten metal plating baths having an extremely longservice life is obtained, and it is possible to greatly extend theperiod of continuous operation during which no equipment is replaced,and this is extremely useful in industry.

We claim:
 1. An equipment for use in a molten metal plating bath,comprising a bearing disposed in the molten metal plating bath forsupporting a roller with an end, said bearing containing 70-95 wt. % ofSi₃ N₄, 2-10 wt. % of Al₂ O₃, 1-10 wt. % of AIN and 1-10 wt. % of Y₂ O₃and having a concave with a rectangular shape in section, said end ofthe roller being directly disposed in the concave to be supported by thebearing without using rollers.
 2. An equipment for use in a molten metalplating bath, comprising a bearing disposed in the molten metal platingbath for supporting a roller with an end, said bearing containing 70-95wt. % of Si₃ N₄, 2-10 wt. % of Al₂ O₃, 1-10 wt. % of AIN and 1-10 wt. %of Y₂ O₃ ; and having a cylindrical shape with a hole, said end of theroller being directly disposed in the hole to be supported by thebearing without using rollers.
 3. An equipment according to claim 2,further comprising a synch roller arm, to which said bearing isattached.
 4. An equipment according to claim 3, further comprising asynch roller thrust lock attached to the synch roller arm and disposednear the bearing.